This application claims priority to German Patent Application No. 101 24 358.8, filed May 18, 2001.
The invention relates to a laser system for ophthalmic surgery, in particular for corneal grafting.
Grafting of a part of the human cornea may be indicated for a variety of reasons. For example, in the case of so-called keratoconus an irregular change occurs in the shape of the cornea, so that the optical imaging suffers too severely. Corneal grafting may also become necessary in the case of extreme clouding of the cornea as a result of, for example, loss of endothelial cells, infections, ulcers, hereditary diseases or scarring.
In a corneal grafting operation the central part of the diseased cornea in the region of the optical axis is removed surgically and a healthy donor graft is inserted.
Removal of the recipient cornea and of the donor cornea is undertaken in the state of the art by so-called trepanation. Mechanical instruments, in particular a so-called manual trepan or motorized trepan, are employed for this purpose in the state of the art. Such trepans according to the state of the art enable removal of disc-shaped corneal segments. The known trepans comprise a circular cylindrical hollow body which is ground at the lower end to form a sharp cutting surface. The diameter of the circular ground edge corresponds roughly to the diameter of the central corneal segment to be excised. In this connection, care has to be taken to ensure that the endothelium is traumatised as little as possible. A further important aspect in corneal grafting is a watertight closure of the wound.
Mechanical corneal surgery with a trepan has the disadvantage, firstly, that the geometrical structure of the excised corneal segment is defined by reason of the predetermined ground edge; by reason of the circular cylindrical configuration of the trepan only circular cylindrical segments are capable of being cut. The tendency towards deflection of the incision in the lamellar tissue depending on the curvature of the cornea also causes problems with regard to precise reproducibility of the incision. Once a trepan has been applied, only the depth of the incision can be controlled. The state of the art in trepan incision is familiar with a variety of processes for this purpose, for example the use of punches in the trepan.
A principal problem in corneal grafting is the liquid-tight closure of the wound. In the state of the art the corneal graft is fixed by the placement of a suture after successful implantation. The suture typically remains in the recipient cornea for a period of up to one year. Such a suture is not only very elaborate, it can also lead to various complications; in particular it can bring about an incorrect optical position of the donor graft or even an insufficient liquid seal. The suture can also change the optical properties of the imaging system constituted by the eye; for example, distortions in the cornea can be induced by the suture, which can lead to an astigmatism.
The liquid seal is a fundamental problem. In the eyeball the human eye has a relative overpressure of about 15 mmHg in the normal case.
The object underlying the invention is to provide means with which corneal grafting can be performed better with regard to the problems of the state of the art outlined above.
For this purpose the invention provides a laser system having the features of claim 1.
A laser system of such a type enables corneal grafting using so-called photodisruptive laser cutting.
The generation of extremely short laser pulses has recently made it possible, in ophthalmological surgery in particular, to separate a corneal lobule without mechanical cutting means by focusing of the laser pulses in the interior of the cornea, for example within the scope of LASIK treatment. Short laser pulses within the nanosecond to femtosecond range have such high power outputs that, with suitable focusing, the biological material can be xe2x80x9ccutxe2x80x9d in its interior without internal thermal effects or such like arising.
By virtue of the undercut in the xe2x80x9cincisionxe2x80x9d of the cornea which is obtained in accordance with the invention, a self-sealing is obtained, since the aforementioned relative overpressure within the eye brings about full contiguity of the implant with the residual cornea of the recipient. Moreover, the shape of the incision that is obtained in accordance with the invention has the advantage that it also promotes the optical centering of the donor implant in the recipient cornea. By this means, the suturing of the inserted implant is also reduced to a minimum and the disadvantages caused thereby are very largely avoided.
According to a preferred configuration of the invention the undercut is zigzag-shaped. The undercut is so designed that, viewed from the interior of the eye, a part of the residual recipient cornea encroaches upon the inserted donor implant radially towards the inside on the outside in relation to the optical axis.
A particularly preferred configuration of the control program of the computer unit for the laser system provides that the focus control brings about an undercut in the cornea which generates an incised segment that extends at least approximately radially in relation to the axis of the cornea. This radial segment generates a sealing surface which, by reason of the internal pressure within the eye, acts optimally as an annular sealing surface. The plane-parallel faces of the implant, on the one hand, and the residual cornea, on the other hand, are juxtaposed fully and over a large area under the overpressure of the eye, and the force brought about by the pressure difference is substantially perpendicular to these sealing surfaces.